The present invention relates, in general, to an injection unit for injection molding machines with continuously operating plasticizing unit.
Injection molding machines are known in which a reciprocating screw equipped with a back flow valve implements the plastification of the plastic granulate and generates the injection pressure. Other approaches involve an operational division between a screw plasticizing unit, on the one hand, and a plunger-type injection unit, on the other hand. Constructions of this type have the advantage that a screw plasticizing unit can operate at optimum plasticizing capacities. Moreover, the use of several plunger-type injection units, fed from a screw plasticizing unit, permits the screw plasticizing unit to be continuously operated because melt can be fed alternating to the various, normally two, plunger-type injection units. A continuous operation of the plasticizing unit is advantageous because not only is the melt more homogenous, but the plasticizing capacity can be increased since shutdown periods are eliminated or a smaller screw can be utilized while maintaining a same capability. Furthermore, wear is significantly reduced, in particular when large screws are involved, because there is no need for a constant re-starting (absence of adhesive wear during start-up). Also the motor and the transmission are subject to less stress in view of the even load.
A screw plasticizing unit in combination with plunger-type injection units can also be used as compounder for mixing varied components. As the plasticizing screw is not moved back and forth, solid and liquid additives can always be fed at the same location relative to the screw. The same is true, when producing foamed products, because foaming agent can also be added always at the same spot. The continuous operation prevents an undesired pressure drop of the foaming agent, as has been experienced with injection molding machines with reciprocating screws during shutdown period.
An injection molding machine with a separate plasticizing unit, on the one hand, and several plunger-type injection devices, on the other hand, suffers however the drawback that the plurality of existing plunger-type injection devices requires significant space for installation. Moreover, long melt channels that need to be heated, and respective pressure losses have to be accepted. Further, the alternating feeding of the plunger-type injection units require reverse valves. Typically, also the purging quality is poor because the cylinder spaces of the plunger-type injection units cannot be filled and emptied according to the principal of a continuous melt stream flowing in only one direction (first-in, first-out).
German Pat. No. DE 195 05 984 A1 discloses an injection molding machine with separate and continuously operating plasticizing unit and two plunger-type injection devices, but with unidirectional melt flow (first-in, first-out). This publication illustrates an injection molding machine in which a screw plasticizing unit, comprised of two plasticizing screws running in a same direction, is connected downstream to two plunger-type injection devices which, controlled by reverse valves, can be fed alternatingly with melt. A unidirectional melt flow is realized in this injection molding machine by supplying each plunger-type injection device through conduction of melt via telescopically movable melt channels into the respective plunger bottom of the plunger-type injection devices. This significantly increases, however, the length of the melt channel.
International patent publication WO86/06321 discloses an injection molding machine for manufacturing injection-molded or extruded formed parts of mixed materials. Examples of mixed materials include hereby plastics with additional components such as fibers, particles, binders or other additives. The plastic granulate is melted in a twin or multiple screw compounder. The additional components are also added to the compounder and mixed with the plastic melt as homogeneously as possible. A melt channel extends from the forward end of the compounder and connects into the rear region of an injection space in which an injection plunger is guided for reciprocating movement. Melt flows through a back flow valve, arranged at the forward end of the injection plunger, and fills the injection space in front of the injection plunger to thereby push the injection plunger backwards. When enough melt fills the injection space, the compounder is stopped and the supply of melt is interrupted. Melt, stored in the injection space, is injected into the injection mold as the injection plunger is moved forward through application of a pressure medium. After filling the mold cavity, an afterpressure plunger, disposed downstream of the injection space and engaging the melt exit channel, which extends to the mold cavity, blocks the melt exit channel, connected to the injection space, and, optionally, to generate an afterpressure. This conventional injection molding machine includes an unidirectional melt flow (first-in, first-out) and avoids long melt paths, when several plunger-type injection devices are involved (like, for example, in German Pat. No. DE 195 05 984 A1). It is, however, disadvantageous that the compounder has to be stopped during the injection phase and afterpressure phase. This adversely affects the melt quality because only a continuous operation of the compounder (or extruder) provides for a homogeneous melt throughout. Moreover, the provision of back flow valves is disadvantageous when processing melts filled with long fibers or natural fibers because a flow through narrow and wound openings in the back flow valve entails a risk of damage of the fibers.
It would be desirable and advantageous to provide an improved an injection unit for injection molding machines to obviate prior art shortcomings and to allow a permanent continuous operation of the plasticizing unit regardless of the cycle time in combination with an intermittent operation of the injection molding process.